1. Field of the Invention
The present invention relates to power and data systems for use in permanent installations in wells such as oil and gas wells. In particular, the invention provides such systems which incorporate redundancy in order to improve overall system reliability.
2. Description of Related Art
It has been known for some time to install permanent monitoring devices and valves in wells such as oil and gas wells in order to monitor and control the operation of the well over its lifetime. Such installations require the provision of power and data communication systems for each device. One such system is the WellNet power distribution and communication system of Schlumberger, which is described in more detail in U.S. Pat. No. 6,369,718.
WellNet uses an armored twisted pair cable to transport both communication and power. In this configuration, shown schematically in FIG. 1, power is sent in common mode, often referred to as the “phantom wire”, and communication is sent in differential mode, on the twisted wires.
FIG. 2 shows a simplified representation of the theoretical principle. The current “Icom” goes from A to B in the primary winding of the surface transformer of the transmitter. The secondary of this transformer sends the modulation in the cable through one wire of the twisted pair with return on the other wire. The signal is picked by the primary of the downhole device transformer, down the cable. The cable armor is never involved in the communication path. Power “Ipow” is sent to the centre tap of the secondary of the transmitter transformer. The current “Ipow/2” flows in the same direction through each wire of the twisted pair. It goes out at the centre tap of the primary of the receiver transformer, enters the power converter of the downhole tool “Load”, and returns through the cable armor. The power supply uses both wires of the twisted pair in parallel, allowing large power transfer capabilities, even when using small wires.
FIG. 3 shows the manner in which the WellNet system is applied to multiple tools connected on the same cable. Each tool (Node) intercepts the twisted wires to the power and data in the manner described above. A termination resistance is located in the lower cable head of the lowermost device in the string to avoid signal reflections. In order to communicate between the surface controller and the downhole tools, a communication protocol is implemented, each node being addressable independently via the network. Each mode will include one or more electronics modules (WellNet modules) with appropriate firmware to manage this activity.
The system described above has a single cable. Therefore, damage to the cable can mean that the whole installation can become inoperative. One way to avoid this possibility is described in WO 00/46616, which shows a loop configuration for the cable. In this case, instead of terminating the cable at its lower end, the cable is returned to the surface of the well and is connectable to another power and data system. Each node can be provided with power and data from either surface supply, and each node is provided with switches to allow this to be selected. In the event of a problem on the cable, typically detected by the loss of signal, the switches at each node are operated until the location of the fault is identified. Following this, the switches on the nodes above the fault are set to take power and data from the original supply, and those of the nodes below the fault to take power and data from the second supply. Thus the effect of the fault can be limited to one or two devices rather than affecting the whole installation. However, this system requires that the cable run to the bottom of the well, and back to the surface, and that two supplies be provided.
It is an object of the present invention to provide a power and data system which does not require a looped cable in order to operate in the event of faults.